The invention relates to a method and an apparatus for coding and decoding a video frame in a telecommunication system, when the video frame is made up of a string of consecutive stationary frames, in which the frames to be transmitted are typically divided into frame blocks or segments, for instance pixel groups, specific for each frame and the data in each frame block typically comprises information indicating the luminance, color and location of the frame block.
Transferring video frames in telecommunication systems, such as video phones, video conference systems or Internet connections, is a demanding task due to the large amount of data needed to transfer a video frame, since the more bits are needed to transfer the data, the higher the data transmission rate must be. Various methods have been developed to solve this problem. To transfer a frame, the frame is usually divided into frame blocks whose size is selected to suit the system. Frame block information generally comprises information on the luminance, color and location of the frame block in the frame itself. The frame block data is compressed by each block using a desired coding method. The compression is based on removing the less significant data. Compression methods are divided primarily into three classes: spectral redundancy reduction, spatial redundancy reduction and temporal redundancy reduction. Various combinations of these methods are typically used in compression.
A YUV color model, for instance, is applied to spectral redundancy reduction. The YUV color model utilizes the fact that the human eye is more sensitive to changes in luminance than in chrominance, i.e. color. The YUV model has one luminance component (Y) and two chrominance components (U, V). The chrominance components are also sub-sampled. For instance, a luminance block according to the H.263 video coding standard is 16xc3x9716 pixels and both chrominance blocks, which cover the same area as the luminance block, are 8xc3x978 pixels. The combination of one luminance block and two chrominance blocks is in this standard called a macro block.
To reduce spatial redundancy, a discrete cosine transformation (DCT), for instance, is used, in which the block to be compressed is preferably 8xc3x978 pixels. In a DCT, the pixel presentation of a frame block is transformed into a space-frequency presentation. In addition, only the signal frequencies which exist in a frame block have high-amplitude coefficients, whereas the coefficients of the signals that do not exist in a frame block are close to zero. DCT is also a lossless transformation and interference is caused to the signal in quantizing only.
Temporal redundancy can be reduced by utilizing the fact that consecutive frames usually resemble each other, so instead of compressing each individual frame, motion data of the frame blocks is generated. The basic principle is as follows: an as good as possible reference block which has been coded earlier is searched for the frame block to be coded, the motion between the reference block and the frame block to be coded is modeled and the calculated motion vector coefficients are transmitted to the receiver. The difference between the block to be coded and the reference block is indicated as a prediction error component or frame. The problem is to find a reference block which produces as good a coding efficiency as possible (a sufficiently good picture quality with as small an amount of bits as possible) and a high calculation capacity, and thus also calculation time, required by the comparison.
It is thus an object of the invention to provide an improved method for coding video frames, an improved video coder and an improved subscriber terminal. The invention is based on using the frame reconstruction data of at least one or more reference frames or reference blocks and of one or more earlier coded neighbouring blocks in coding the frame reconstruction data. The frame reconstruction data and motion data coded with the method of the invention or with another known method is selected for use depending on how the desired coding efficiency is obtained with a sufficient picture quality.
This is achieved by a method of coding video frames in a telecommunication system, comprising: forming a video frame of consecutive stationary frames, storing the frame reconstruction data of at least one frame as a reference frame and the motion data of earlier coded neighbouring blocks, defining by means of the motion data of one or more earlier coded neighbouring blocks the motion data of the block to be coded, which neighbouring block is formed by means of the stored reference frame, defining the frame reconstruction data of the frame to be coded, selecting for use the frame reconstruction data and motion data representing the block to be coded, which provide a pre-defined coding efficiency with a predefined picture quality.
This is achieved by a method of coding video frames in a telecommunication system, comprising: forming a video frame of consecutive stationary frames, storing the frame reconstruction data of at least one frame as a reference frame and the motion data of earlier coded neighbouring blocks, defining the motion data of a frame block to be coded using only the neighbouring blocks which have the same reference frame used to code the frame reconstruction data of the block to be coded, selecting for use the frame reconstruction data and motion data, which provide a pre-defined coding efficiency with a pre-defined picture quality.
The invention also relates to a subscriber terminal comprising: means for forming a video frame of consecutive stationary frames, means for storing the frame reconstruction data of at least one frame as a reference frame and the motion data of earlier coded neighbouring blocks, means for defining by means of the motion data of one or more earlier coded neighbouring blocks the motion data of the block to be coded, which neighbouring block is formed by means of the stored reference frame, means for defining the frame reconstruction data of the frame to be coded, means for selecting for use the frame reconstruction data and motion data representing the block to be coded, which provide a pre-defined coding efficiency with a predefined picture quality.
The invention also relates to a subscriber terminal comprising: means for forming a video frame of consecutive stationary frames, means for storing the frame reconstruction data of at least one frame as a reference frame and the motion data of earlier coded neighbouring blocks, means for defining the motion data of a frame block to be coded using only the neighbouring blocks which have the same reference frame used to code the frame reconstruction data of the block to be coded, means for selecting for use the frame reconstruction data and motion data, which provide a pre-defined coding efficiency with a pre-defined picture quality.
The invention also relates to a video coder comprising: means for forming a video frame of consecutive stationary frames, means for storing the frame reconstruction data of at least one frame as a reference frame and the motion data of earlier coded neighbouring blocks, means for defining by means of the motion data of one or more earlier coded neighbouring blocks the motion data of the block to be coded, which neighbouring block is formed by means of the stored reference frame, means for defining the frame reconstruction data of the frame to be coded, means for selecting for use the frame reconstruction data and motion data representing the block to be coded, which provide a pre-defined coding efficiency with a predefined picture quality.
The invention also relates to a video coder comprising: means for forming a video frame of consecutive stationary frames, means for storing the frame reconstruction data of at least one frame as a reference frame and the motion data of earlier coded neighbouring blocks, means for defining the motion data of a frame block to be coded using only the neighbouring blocks which have the same reference frame used to code the frame reconstruction data of the block to be coded, means for selecting for use the frame reconstruction data and motion data, which provide a pre-defined coding efficiency with a pre-defined picture quality.
The invention also relates to a computer program which can be read by a computer and which executes the method of coding video frames in a telecommunication system, comprising: forming a video frame of consecutive stationary frames, storing the frame reconstruction data of at least one frame as a reference frame and the motion data of earlier coded neighbouring blocks, defining by means of the motion data of one or more earlier coded neighbouring blocks the motion data of the block to be coded, which neighbouring block is formed by means of the stored reference frame, defining the frame reconstruction data of the frame to be coded, selecting for use the frame reconstruction data and motion data representing the block to be coded, which provide a pre-defined coding efficiency with a predefined picture quality.
The invention also relates to a computer program which can be read by a computer and which executes the method of coding video frames in a telecommunication system, comprising: forming a video frame of consecutive stationary frames, storing the frame reconstruction data of at least one frame as a reference frame and the motion data of earlier coded neighbouring blocks, defining the motion data of a frame block to be coded using only the neighbouring blocks which have the same reference frame used to code the frame reconstruction data of the block to be coded, selecting for use the frame reconstruction data and motion data, which provide a predefined coding efficiency with a pre-defined picture quality.
The method and apparatus of the invention provide savings in calculation time and capacity while the picture quality remains adequate.